1. Technical Field
The invention relates to optical communications links, in particular optical free-space communications links for transmission over short distances.
2. Related Art
Recent advances in consumer electronics have led to the development of many different devices such as digital cameras, personal digital organisers, MP3 players, etc that are capable of exchanging data directly with personal computers (PCs) or over communications networks, for example, downloading data from World Wide Web (WWW) servers. Many of these use connectors complying with the USB standard.
A USB connection is a half-duplex communication, carried over two data wires. An additional two wires carry power. The two data wires do not have a data flow direction associated with them, as the RS232 serial communication does, but the two wires give a differential signal whose relative polarity defines a logic zero or one. Hence, the data must be sent in a half-duplex fashion to prevent collisions on the connection, which of course would lead to data corruption.
Users of devices connected by a USB connection can use a connection to a PC (personal computer) or to a communications network to transfer data to or from their device (for example transferring digital camera image files to a WWW server or synchronizing diary data held on a PDA (personal digital assistant) with that held on a PC). It is relatively easy for users to connect their devices to transfer data when in, for example, their home or their workplace where there is access to a communications network, for example using dial-up access over the PSTN (public switched telephone network) or access to a LAN (local area network)(with an Internet gateway.
Several multi-wire schemes are available such as the standard Printer Parallel Bus and the IEEE488 GPIB Bus, hard-wired serial schemes such as the RS232 Serial Bus, and optical serial schemes like the IrDA Optical Bus. These methods all require complex software control systems to allow data transfer across the link with no intervention from the systems whatsoever. Any error handling or flow control comes from the application software itself.
If users do not have such network access then the fallback option is to connect using a mobile communications network. Whilst this may be an acceptable choice when sending short emails or synchronising a diary from a PDA, the limited transmission capabilities (9.6 kb/s with a GSM mobile telephone network) make it impractical for exchanging larger quantities of data, for example uploading digital camera images or downloading music files when the respective devices may have memory card storage capacities of 256 MB or even greater (very much greater for devices having disk storage capabilities). Although GPRS and UMTS mobile communication networks promise to deliver greater transmission rates to mobile users they are not currently in use and it is not altogether clear what data transmission rates will be available to users in practice.